Antioxidants are highly beneficial to human health, and their accumulation in lettuce, one of the most popular leafy vegetables, depends on both genetic and environmental factors. Nitrogen (N) availability plays an essential role in regulating antioxidant accumulation, but the influence of genotype × N interactions on the antioxidant qualities of lettuce is poorly understood. Therefore, the present study investigated the variation of growth and antioxidant qualities of 20 lettuce (Lactuca sativa L.) genotypes (10 green lettuce genotypes and 10 red lettuce genotypes) under limited N (low N, LN) conditions and standard N (high N, HN) conditions. For all 20 genotypes, LN conditions reduced shoot (i.e., leaf) growth, but increased plant concentrations of vitamin C, glutathione, and phenolic compounds, with the exception of carotenoids, compared with HN conditions. Because of reduced biomass under LN conditions, not all lettuce genotypes exhibited increased antioxidant yields or total antioxidant capacity yield. The variation in antioxidant quality was primarily genetically determined. Generally, the green lettuce genotypes exhibited more pronounced increases in antioxidant yields and total antioxidant capacity yield than the red lettuce genotypes under LN conditions. These results suggest that even though LN conditions generally tend to improve the antioxidant qualities of lettuce, the extent of this effect is highly dependent on genotype. Therefore, genotype should be given priority in future studies that aim to improve antioxidant qualities in lettuce through N management.
